Field of the Invention
The invention relates to jitter and more particularly to dealing with jitter that contains large systematic jitter relative to random jitter.
Description of the Related Art
Optical transport networks (OTNs), broadcast video, and other applications use timing signals as part of the system to transport payloads. Such timing signals include a signal component caused by random or thermal related jitter. Thus, timing signals for one part of the system are nominally, but not exactly the same as timing signals for another part of the system. Phase-locked loops (PLLs) have traditionally been used to deal with the random jitter. However, transport networks may also insert systematic jitter in timing signals by, e.g., inserting gaps in clocks to align input and output data. For example, assume data is being received at a network node at a rate of 1 Gb/s but is being transmitted from the node at a slower rate of 1% less than 1 Gb/s. One way to deal with that rate difference is to skip pulses or insert gaps into the timing signal (nominally 1 Gb/s) transmitted with the slower transmitted data. Thus, skipping clock pulses can be used to account for slightly different input and output data rates.
Use of gapped clocks is a convenient technique used in communication systems to pass timing and frequency information. The technique has the advantage of being a simple and universal interface where frequency/timing information is embedded within the clock signal. The drawback of using the gapped clock technique, however, is the jitter caused by the gaps for the downstream system. Since the gaps are inserted by OTN mappers/de-mappers, there is no noise shaping, nor any pattern control of these gap insertions. To reduce the clock jitter for downstream systems, very low bandwidth jitter cleaning devices (e.g. below 10 Hz) are typically used to filter out the jitter/wander caused by clock gaps. Since the gap patterns are very unpredictable and difficult to model and characterize, system performance cannot be guaranteed. That is one reason gapped clock techniques are not widely used despite the cleanness in system partitioning afforded by gapped clocks. Other drawbacks of gapped clock use is that low bandwidth jitter cleaning, which is sensitive to temperature fluctuations, has excessive system response latency. In addition, gapped clock use increases system cost due to the need for very low bandwidth jitter cleaning devices.